Phase shifting of the intake and exhaust events of an engine causes the intake valve to close later or earlier than normal in the engine cycle to permit unthrottled operation at all or most part load conditions to reduce the engine pumping losses and obtain other output benefits. The air flow to the engine then is controlled by varying the volume of air trapped in the cylinder, which is accomplished by changing the timing of the late or early intake valve closing. As a result, the clearance volume should also be varied concurrently with the changes in trapped air volume if the desirable schedule of compression ratio as a function of engine load is to be maintained. This invention relates to an apparatus for changing the stroke clearance volume of an engine.